1. Field of the Invention
The present invention relates to an outboard motor to be mounted in a boat.
2. Description of the Related Art
In general, an outboard motor mounted in a boat has an upper casing and a lower casing, and an engine is disposed in the upper casing. An exhaust passage connected to a plurality of cylinders in the engine is disposed to extend from the inside of the upper casing to a bottom portion of the lower casing. The exhaust passage is provided with a catalyst that purifies exhaust gas.
In such a construction, exhaust gas flowing out from each cylinder to the exhaust passage is purified in the catalyst, and then discharged into water from the bottom portion of the lower casing.
A lower end portion of the exhaust passage is immersed in water. Therefore, water in the lower end portion of the exhaust passage may flow backward to an engine side as a result of negative pressure or the like that is generated in the engine. Especially, a four-stroke engine is largely affected by exhaust pulsation, so water is sucked into an engine side by strong force in the exhaust passage.
In order to prevent deterioration of the catalyst, water flowing backward in the exhaust passage must be prevented from adhering to the catalyst. To prevent water adhesion to the catalyst, an outboard motor in which a catalyst is disposed in an upper casing has been developed (for example, refer to JP-A-2000-356123).
However, even if a catalyst is simply disposed in the upper casing as indicated in JP-A-2000-356123, water adhesion to the catalyst cannot be sufficiently prevented when water flows backward in the exhaust passage. In addition, with the construction of the outboard motor described in JP-A-2000-356123, the catalyst must be disposed at an even higher position to securely prevent water adhesion to the catalyst. Accordingly, the size of the outboard motor is disadvantageously increased.